The actinic energy radiation curable composition which can be cured by the activity energy radiation such as ultraviolet radiation and an electron beam is put in practical use by various uses such as paints for plastics, paper, woodwork, and inorganic materials, adhesives, printing inks, printing circuit board, and electric insulation.
As an inkjet ink system using these polymerizable composition, proposed is an ultraviolet radiation curable inkjet ink curable by an ultraviolet radiation. The inkjet system using the ultraviolet radiation curable ink is attracting attention in recent years in view of ensuring high abrasion resistance and adhesion to the non-ink absorbable recording medium.
However, when the high speed recording is performed such as a single pass recording mode using the line recording head or a high speed serial mode in small passes by the image forming method using these ultraviolet radiation curable inkjet system, the coalescence of dots adjacent each other could not be inhibited, and caused a problem of inferior image quality. Moreover, when a color image was recorded, color mixture occurred between colors and causes a problem of image quality deterioration.
As a method for resolving the above-mentioned problems, disclosed was a technology which contained a gelling agent in an ink and used an actinic energy radiation curable ink which had a characteristics of solid/liquid phase transition depending on temperature, thereby solidified an ink droplet simultaneously with the deposition of the droplet, resulted in preventing the coalescence of ink droplets and degradation of the image quality. However, this method sometimes caused an extreme decrease of glossiness in an image depending on physical properties of the ink or printing conditions, or an unnatural image quality without sense of homogeneous by glitter in the image.
For example, Patent Document 1 discloses a method of forming an image using an inkjet ink containing a curable gelling agent and a curable monomer. This Patent Document is characterized by using so called elastic gel ink which has high viscosity of 1.0×106-1.0×109 mPa·s at a recording temperature, and storage elastic modulus G′ being larger than loss elastic modulus G″. In the case of recording by using such elastic gel ink, since the droplet was solidified too strong, fine unevenness arose on the image surface and causes problems such as decrease of glossiness or the unnatural glitter in the image.
For example, Patent Document 2 discloses an image forming apparatus using an inkjet ink containing a photo-curable monomer and a gelling agent and recording was performed at a temperature of a substrate being 5° C. or more lower than a gelation temperature. However, this Patent Document aimed at the improvement in curability, and a method for arranging the homogeneous glossiness was not indicated at all. Moreover, in the case of using the low molecule gelling agent as used in this patent document which formed a fiber structure, there was a case where gelation temperature became excessively higher than needed depending on the kind of ink, or the ink after gelling became hard and the decrease of glossiness was observed since the droplet was solidified too strong in the above-mentioned temperature range.
Moreover, Patent Document 3 discloses a curable ink characterized in that the glossiness of a printed region was closely matching the glossiness of a substrate, however the specific means how to match the glossiness was not indicated. Moreover, the ink in this Patent Document had the viscosity of 1.0×105-1.0×107 mPa·s at the recording temperature of 50° C. or less. The ink had strong solidification strength to the extent of suppressing ink penetration into a plain paper. Therefore, unless applying the intermediate transferring system, there caused a problem of the extreme decrease of glossiness or glitter in the image resulting from dot solidification strength being too strong.
Moreover, Patent Document 4 discloses a technology using a thixotropic curable gel ink in which the recording was performed so that the temperature difference between the ink at ejection and the recording medium became 30° C. or more. When the gelation temperature of the photo curable ink was high or when the ink after gelling became hard, the decrease of glossiness was observed which was resulted from the dot solidification strength being too strong in the above-mentioned temperature range.
Moreover, Patent Document 5 disclosed an attempt to increase the image glossiness by heating an over-coating liquid containing a gelling agent to the same or higher temperature than a phase transition temperature during the time from the deposition on the recording medium to the UV radiation irradiation. It was satisfactory in the case of using the overcoat liquid. However when the ink containing a gelling agent and a colorant for forming the image was used as the same manner, the deteriorated image quality such as color mixture occurred and it had become a problem.